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One of the most important numbers in physics, the proton-electron mass ratio, is the same in a galaxy six billion light years away as it is here on Earth, according to new research, laying to rest debate about whether the laws of nature vary in different places in the Universe.

"We have been able to show that the laws of physics are the same in this galaxy half way across the visible Universe as they are here on Earth," says Michael Murphy, lead author of the study.

The astronomers determined a proton-electron mass ratio of 1836.15 by making spectroscopic observations of distant quasar B0218+367, located 7.5 billion lights away, with the Effelsberg 100 metre radio telescope. On its journey to us, the quasar’s light was partially absorbed by ammonia gas in an intervening galaxy located at distance of six billion light years. The wavelengths at which ammonia absorbs radio energy from the quasar are sensitive to the proton-electron mass ratio.

"By comparing the ammonia absorption with that of other molecules, we were able to determine the value of the proton-electron mass ratio in this galaxy, and confirm that it is the same as it is on Earth," says Christian Henkel from the Max Planck Institute for Radio Astronomy.

Observing molecular absorption in the distant galaxies using the light of bright background quasars. As light from the quasar travels to Earth, the Universe continues to expand, stretching the light’s wavelength. In the observations, the light is also gravitationally lensed (bent) as it passes through the intervening galaxy. Molecular absorption images are made along the line of sight of the quasar, revealing structure details such as a core and a ‘knotty jet’ extending away from the core. Image:
Telescope: N. Junkes; Radio insets: A. Biggs; Intervening galaxy: NASA, ESA, STScI & W. Keel; Quasar: NASA, ESA, STScI & E. Beckwith.

So far, B0218+367 is the only target for this kind of research. The astronomers plan to continue testing the laws of nature in as many different places and times in the Universe as possible, in order to see how well they stand up in untested situations. Every new measurement of this value will be an invaluable test of the most basic – and theoretically unjustifiable – assumptions in physics, including those that state that the laws of physics are universal and unchanging. The precision demonstrated by this particular study highlights the importance of discovering many more absorbing galaxies to further our knowledge of fundamental physics.

In order to locate more suitable galaxies, Murphy thinks that the Square Kilometre Array (SKA) will be the perfect tool. "The SKA is the largest, most ambitious international telescope project ever conceived," he says. "When completed it will have an enormous collecting area, and will allow us to search for more absorbing galaxies." The location of the SKA, which has been short-listed to Western Australia or Southern Africa, will be announced within the next two years.

By continuing their research into the forces of nature, the
astronomers also hope to find a window into the extra dimensions of space that many theoretical physicists think may exist.

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